Field of the Invention
The present invention relates generally to fuel assemblies for a nuclear reactor and, more particularly, to an advanced method of fuel management through arrangement of nuclear fuel assemblies within the initial core of a pressurized water reactor. The invention also relates to advanced initial cores for pressurized water nuclear reactors.
Background Information
Modern commercial nuclear power reactors are fueled with uranium having a slightly enriched U-235 content. The core of the reactor is formed by numerous elongated, rectangular fuel assemblies arranged in a cylindrical vessel. The fuel assemblies are arranged in accordance with a loading pattern intended to meet certain engineering requirements, such as the distribution of power, including limits on power peaks within the core. Other considerations include the maximization of the fuel cycle, or the time required between refuelings. The initial loading configuration and plan of replacement and arrangement of fuel during the life of the reactor is known as in-core fuel management, and is a major nuclear reactor design consideration. Use of the slightly enriched U-235 fuel necessitates that portions of the core be periodically removed and replaced with new or fresh fuel. Thus, it is common to combine fuel assemblies from previous fuel cycles with new fuel. A typical inventory of fuel assemblies includes about one-third new fuel assemblies, commonly referred to as feed assemblies, about one-third once-burned fuel assemblies and about one-third twice-burned fuel assemblies.
Accordingly, the fuel loading pattern for the first core of a nuclear reactor, such as a pressurized water reactor, commonly utilizes three enrichment zones, based upon the average enrichment of U-235 of the fuel assemblies with a given zone, with each zone having generally equal proportions. FIG. 1 shows a schematic representation of such a known prior art loading pattern 2 which uses three generally equal-sized fuel batches 4,6,8. A batch is a group of fuel assemblies that are typically placed into, and then permanently removed from, the core 14 together. Note that the pattern shown in FIG. 1 illustrates only one-eighth of the reactor core 14 and assumes core symmetry. The zones include two low enrichment zones 4, 6 which, as shown, are generally loaded in a checkerboard fashion toward the reactor interior 10, and a high enrichment zone 8, which is loaded primarily at the reactor periphery 12.
The length of the fuel reload cycle for cores 14 assembled in accordance with the foregoing arrangement is adjusted by varying the enrichments of all of the zones 4, 6, 8 in a substantially equal manner. However, this results in the disadvantage of relatively poor fuel utilization due primarily to high neutron leakage caused by the high enrichment zone 8 being disposed at the reactor periphery 12. Additionally, the use of three generally equal-sized regions in zones 4, 6, 8 is inconsistent with current industry fuel management practices with regard to reload cycles, wherein the refueling fraction varies as a function of the desired cycle length. A cycle is the time during which the arrangement of normally stationary fuel in the reactor core is unchanged, usually beginning with the placement of a feed batch, or a batch of fresh fuel, into the core, and ending with the removal of highly burned fuel assemblies. The number of burns a fuel assembly has experienced is the number of cycles it has been in the reactor core. A typical cycle might range from 10 to 18 months in duration. By way of example, 18-month cycles in accordance with the aforementioned prior art require about 40% of the core to be replaced at each cycle, with the replacement typically comprising a mixture of fuel assemblies having both low and high initial enrichments. Therefore, it will be appreciated that the use of equal size batches results in the discharge of low enrichment regions or zones at very low burn-up, while requiring a significant financial investment. Accordingly, such practice is inefficient and uneconomical. The greatest savings in overall fuel costs is achieved by minimizing the initial enrichment required to achieve an equilibrium fuel management scheme.
Further adding to the inefficiency of known prior art methods for establishing initial cores is the fact that such methods are essentially ad hoc basis, performed on a substantially trial and error basis relying on years of experience in the art. There is no systematic approach for developing the first core. As such, initial cores created by such methods must be conservatively designed, with a built in margin for error, which results in the core taking longer than necessary to reach equilibrium, thereby raising costs.
It is desirable, therefore, to avoid the substantial fuel cycle cost penalties associated with known initial core nuclear fuel management schemes. There is a need, therefore, for an advanced method of implementing initial cores for nuclear reactors.
Accordingly, there is room for improvement in the art of in-core fuel management for nuclear reactors including initial core fuel assembly arrangement and in methods of implementing the same.